One important characteristic of certain materials, such as plastic barrier films, is the degree to which certain substances, such as gas and vapors, permeate the materials. In certain applications, such as use in photovoltaic electronic devices, low permeation rate to water vapor results in higher shelf life and thus contributes to lower cost. Water vapor transmission rate (WVTR) is a widely used measurement for determining the barrier properties of a plastic film. It is a measure of the amount of water vapor that can permeate through a certain area of a film over a certain period of time. WVTR is one of the key properties in photovoltaic, organic light emitting diodes (OLED) and other electronic devices. One target for commercialization and longevity is water vapor permeation rate in the range of 10−6 g/(m2-day).
Systems, such as the Mocon Aquatran, use a coulometric phosphorus pentoxide sensor that converts water vapor to an electrical charge. Those systems can detect moisture down to the level of 5×10−4 g/(m2-day). Below this level, the coulometric technique is inadequate.
Some laboratory methods, such as the so-called calcium method, exist that can measure water vapor below 10−4 g/(m2-day) using the optical transmission or the electric conductivity of a calcium coating which is encapsulated within a cell that is sealed with the barrier sample of interest. The calcium method, however, is typically laborious and the use of the method has not yet been standardized. Furthermore, the degradation of calcium is a function not only of the permeation rate of water vapor but also due to permeation of other species especially oxygen. In addition, the evidence to date does not confirm that the calcium method can actually measure WVTR as low as 10−6 g/(m2-day). Thus, these systems are not sufficient for measurement of barrier properties for ultra-barriers designed for photovoltaics, OLED and other electronic devices.
Therefore, there exists a need for a gas permeation measurement technique which is simple, easy to use, has the ability to analyze for a specific molecule such as water, and has a low detection limit.